The purpose of the extension of the study is to examine by means of tests of intelligence, achievement, visual and motor skills, and memory and language 90 hypothyroid patients eight to nine years of age who were diagnosed by neonatal screening. At present these children do not differ in intelligence (WISC-R) from control children who include normal siblings, children with thyroxine binding globulin (TBG) deficiency, and children who had transiently low T4 concentrations shortly after birth but whose TSH concentrations were normal. In addition, the patients did not differ from the controls when tested by the Vineland Social Maturity Scale and on the Berry-Buktenica test of visual and motor integration. Examination of 56 of these patients at six years of age disclosed no defects with the single exception of slowness of fine motor movement. It has been reported that children with infantile hypothyroidism, despite normal IQs, have neuropsychological deficits. Consequently, by performing the examination on our children after the eight - ninth birthday, we will be able to administer tests which should define these handicaps, if they do exist, and to correlate them with various clinical features of disease and treatment. In addition to the formal neurological testing by the end of the proposed extension of the study, there should also be data available on school achievement through grades 2 - 3 on all patients. Thus a complete definition of the effects of very early treatment afforded by neonatal screening on the central nervous system of patients with infantile hypothyroidism will be possible. In addition, an analysis of factors affecting the results, which has been made for the outcome of global IQ, will be carried out. The previous analysis has defined the adequacy of treatment more closely than has heretofore been possible. Our future analysis may have even greater clinical applicability because of the refinement in the instruments being used to evaluate the children. The results of the testing of our patients at ages eight through nine will allow pediatricians to provide parents with a more complete prognosis, including any compensatory education programs, if necessary, when children are found to have hypothyroidism.